class DemoPlainClass:
    a: int           # <1>
    b: float = 1.1   # <2>
    c = 'spam'       # <3>


d1 = DemoPlainClass()
# print(d1.a)
print(d1.b)
print(d1.c)

# 这类没有成员！只有static！
print(id(d1.c) == id(DemoPlainClass.c))
print(id(d1.b) == id(DemoPlainClass.b))
# 可以让它有
d1.b = 2
print(id(d1.b) == id(DemoPlainClass.b))